1. Field of the Invention
The present invention relates to a focus adjusting apparatus and a focus adjusting method for use in a variety of video cameras, etc.
2. Description of Related Art
In recent years, video apparatuses such as video cameras have made remarkable progress and are ordinarily provided with an automatic focusing control function, an automatic iris control function, a zooming function, etc., so that the improvements of operability and the increase of functions have been attempted in every part thereof.
With regard to an automatic focusing apparatus, the mail trend of nowadays is to use such a method that focus adjustment is performed by detecting, as an AF evaluation value, the sharpness of an image plane from within a video signal obtained by photoelectrically concerting an object image with an image sensor or the like, and controlling the position of a focusing lens in such a way as to make the AF evaluation value maximum.
The above-mentioned AF evaluation value is obtained generally obtained by using the level of a high-frequency component of video signal extracted by a band-pass filter for a certain pass band, or the like. In a case where an ordinary object is taken for image pickup, the AF evaluation value becomes larger accordingly as a picked-up image becomes sharper, as shown in FIG. 2, and a point at which the level of the AF evaluation value becomes maximum is determined to be an in-focus point.
In the case of a practical video camera, as shown in FIG. 1, an AF microcomputer 115 performs an automatic focus adjusting operation by causing a focusing lens 105 to move in such a way as to make the output signal level of an AF evaluation value processing circuit 114 maximum.
Then, as shown in FIG. 3, the AF microcomputer 115 performs a minute driving action on the focusing lens 105, and if an in-focus state is determined to have been attained, performs an in-focus processing operation. When no in-focus state is determined to have been attained, if a direction according to which the AF evaluation value becomes larger has been determined, the AF microcomputer 115 drives the focusing lens 105 in that direction, i.e., performs the so-called hill-climbing driving action. When the focusing lens 105 has reached such a position as to correspond to the peak of the AF evaluation value, the AF microcomputer 115 again performs the minute driving action on the focusing lens 105. After that, if an in-focus state is determined to have been attained, the AF microcomputer 115 makes a comparison between the AF evaluation value obtained at that time and the newest AF evaluation value. If a difference between the two AF evaluation values is found to be greater than a predetermined level, the AF microcomputer 115 decides that the driving of the focusing lens 105 be re-started, and performs the minute driving action on the focusing lens 105.
The conventional minute driving action will be described below with reference to FIG. 9. Referring to FIG. 9, which is a flow chart showing a processing operation of the AF microcomputer 115 for the minute driving action, the processing operation starts in step S901. In step S902, an AF evaluation value is taken in from the AF evaluation value processing circuit 114. In step S903, a check is made to find if the AF evaluation value taken in step S902 is larger than an AF evaluation value taken in for the last time. If not, i.e., if the AF evaluation value taken in step S902 is smaller than the AF evaluation value taken in for the last time, the flow proceeds to step S904. If so, i.e., if the AF evaluation value taken in step S902 is larger than the AF evaluation value taken in for the last time, the flow proceeds to step S905. In step S904, the focusing lens 105 is driven by a predetermined amount in a direction reverse to the direction in which the focusing lens has been driven for the last time. On the other hand, in step S905, the focusing lens 105 is driven by the predetermined amount in the same direction as the direction in which the focusing lens 105 has been driven for the last time.
In step S906, a check is made to find if the direction for driving the focusing lens 105 has been determined continuously for a predetermined number of times to be the same direction. If so, i.e., if the focusing lens 105 has been driven in the same direction continuously for the predetermined number of times, the flow proceeds to step S907. If not, i.e., if the focusing lens 105 has not been driven in the same direction continuously for the predetermined number of times, the flow proceeds to step S908. In step S908, a check is made to find if the focusing lens 105 remains in the same area for a predetermined period of time, i.e., if the lens position stays within a predetermined range for a predetermined period of time. If so, the flow proceeds to step S909. If not, the flow proceeds to step S910 to bring the processing operation to an end. In step S907, it is considered that the determination of the direction has been attained, and, then, the flow proceeds to step S910 to bring the processing operation to an end. In step S909, it is considered that the determination of an in-focus state has been attained, and, then, the flow proceeds to step S910 to bring the processing operation to an end.
As described above, while repeatedly performing the decision of re-starting→the minute driving→the hill-climbing driving→the minute driving→the decision of re-starting, the AF microcomputer 115 causes the focusing lens 106 to move in such a way as to always make the AF evaluation value maximum.
However, the above-described conventional focus adjusting apparatus has the following drawbacks.
Since the AF evaluation value is generally the level of a high-frequency component of a video signal extracted by a band-pass filter having a certain pass band, the peak level of the AF evaluation value obtained at an in-focus point does not become constant with respect to individual objects for image pickup.
Thus, there is a possibility that, even if the AF evaluation values for the respective objects are the same value, one object is in focus and the other object is out of focus.
Therefore, for example, when the luminance level of an object has changed, even if the AF evaluation value is increasing with the focusing lens being moved, in some cases, an image of the object is blurred in reality. Then, if the focusing lens remains being moved in the same direction, considering that the AF evaluation value is increasing, the image of the object would become conspicuously blurred.